1. Field of the invention
This invention relates to a Sn—Zn—In—Sb based electroconductive bonding material and particularly a Sn—Zn—In—Sb based electroconductive bonding material which can directly adhere to an inorganic nonmetal such as glass or a ceramic. In the present invention, a bonding material means a material which maintains its adhesive force after being bonded to the surface of a substrate, but which can intentionally be peeled off without contaminating the bonded interface when it is subjected to specific treatment such as heating. An electroconductive bonding material according to the present invention is an alloy, so it is bonded to the surface of a substrate by melting in the same manner as solder.
2. Background Art
Among lead-free solder alloys which have been put to practical use in recent years, Sn—Ag—Cu solder alloys are widely used. A Sn—Ag—Cu solder alloy is bonded to an electrode by producing mutual diffusion of Sn in the solder alloy and Cu in the electrode and forming an intermetallic compound. Because this type of conventional lead-free solder alloy performs bonding to a Cu electrode through the formation of an intermetallic compound, it has a high bonding strength.
There is a demand for decreases in the size of recent electronic equipment, and electronic parts used in such equipment are also required to be reduced in size. Due to decreases in the size of electronic parts, it is also necessary to decrease the size of solder joints. Therefore, the amount of a solder alloy used in solder joints is decreased, and the bonding strength of solder joints decreases. Accordingly, there is a desire for an increase in the bonding strength of solder joints. Depending upon the electronic equipment, the material of a portion to which a solder joint is to be bonded is not a metal such as Cu or Ni but is sometimes an inorganic nonmetal such as glass or a ceramic. A high bonding strength is of course required even when soldering is performed to an inorganic nonmetal.
With a conventional Sn—Ag—Cu solder alloy, when the material of a portion to be bonded is an inorganic nonmetal such as glass or a ceramic, the elements constituting the inorganic nonmetal and the elements constituting the solder alloy do not undergo mutual diffusion, so there is no substantial formation of an intermetallic compound. As a result, it is difficult to join an Sn—Ag—Cu solder alloy to an inorganic nonmetal. Even if it is attempted to bond an Sn—Ag—Cu solder alloy to an inorganic nonmetal while applying ultrasonic waves, the Sn—Ag—Cu solder alloy easily peels off the portion to which bonding took place.
A Sn—Zn solder alloy is sometimes used as a lead-free solder for bonding to an inorganic nonmetal. Compared to a Sn—Ag—Cu solder alloy, a Sn—Zn solder alloy has relatively good wettability with respect to an inorganic nonmetal. However, Sn and Zn scarcely form a solid solution with each other, and each element forms a phase of the individual element in a solder alloy. Therefore, when an external stress is applied, stress easily concentrates in the Sn phase, and the strength of the solder alloy decreases, leading to a deterioration in bonding strength.
As an example of a countermeasure to such a deterioration in bonding strength, increasing bonding strength to an inorganic nonmetal by using a soldering iron which applies ultrasonic waves and adding In, Sb, or the like to a Sn—Zn solder alloy is being investigated in Patent Document 1.